Widespread occurrence of the tfd-II genes in soil bacteria revealed by nucleotide sequence analysis of 2,4-dichlorophenoxyacetic acid degradative plasmids pDB1 and p712

Abstract

Variovorax sp. strain DB1 and Pseudomonas pickettii strain 712 are 2,4-dicholorophenoxy-acetic acid (2,4-D)-degrading bacteria, which were isolated from agricultural soils in Republic of Korea and USA, respectively. Each strain harbors a 2,4-D degradative plasmid and is able to utilize 2,4-D as the sole source of carbon for its growth. The 2,4-D degradative plasmid pDB1 of strain DB1 consisted of a 65,269-bp circular molecule with a G+C content of 66.23% and had 68 ORFs. The 2,4-D degradative plasmid p712 of strain 712 was composed of a 62,798-bp circular molecule with a 62.11% G+C content and had 62 ORFs. The plasmids pDB1 and p712 share significantly homologous 2,4-D degradative genes with high similarity to the tfdR, tfdB-II, tfdC-II, tfdD-II, tfdE-II, tfdF-II, tfdK and tfdA genes of plasmid pJP4 of Alcaligenes eutrophus isolated from Australia. In a phylogenetic analysis with trfA, traL, and trbA genes, pDB1 belonged to IncP-1β with pJP4, while p712 belonged to IncP-1ε with pKJK5 and pEMT3. The results indicated that, in spite of the differences in their backbone regions, the 2,4-D catabolic genes of the two plasmids were closely related and also related to the well-known 2,4-D degradative plasmid pJP4 even though all were isolated from different geographic regions. Other similarities in the genetic organization and the presence of IS1071 suggested that these catabolic genes may be on a transposable element, leading to widespread occurrence in soil bacteria.